This could be great. Most taboo stories proceed linearly, showing the incestuous relationship develop. By having one already underway at the beginning, you jump into the action. Makes the audience curious in how they got like that. As the mother and daughter are binding the brother, trying to make him as perverted as they, you can reward the reader by flashing back and exploring the mother-daughter relationship's origin.

As the daughter binds her brother's hand, she remembers the first time her mother did the same to her. She can remember how good it felt being restrained, and having her sexuality be rewritten or retrained by her mother. Maybe the mother and daughter talk about their seduction of one another as they work to convert the son. In fact, talking about it helps break the son, his mom and sister painting mental pictures so vivid he can't stop himself from giving in.

Stressing the mother and sister's fear after brother finds out would make the story more intense and urgent. They need for him to join their circle because otherwise he'll tell the father. This fear pushes them beyond their own (albeit already perverse) comfort zone, makes even them do things they'd never thought about before.

More sequel potential if you add an extra sibling. This first chapter of course ends with the dad coming home none the wiser, and the sister and mom acting as normal as possible even though the son's cum is inside the both of them.

I had a similar concept with "Mom's Oral Servitude." In that story, the son sees his mom eating the daughter's pussy. The mom is mortified that they were caught, but explains that it happened by accident. The mom tried comforting the daughter one night, and she did the only thing she knew how to do in order to make the daughter feel better.

For a story like this, how about the daughter is eager to have the guy join, but the mom is apprehensive because she doesn't want the whole thing to get out of hand. The daughter discovered mom's secret fetish and demanded to join.

The daughter in this story is one of those outgoing girls who talks to much. The mom is prim & proper. The son is your average guy.

Mom agrees to let the son join, and the sister directs the whole thing. Perhaps the mom is tied up while the son spanks her or whatever. Then it goes to oral sex.